Millions of American bats have died since 2006 from a fungal infection known as white nose syndrome. Caused by the fungus Geomyces destructans, amongst some species it has a mortality rate of 95%, but its origins and mechanism aren't fully understood. It's thought that the fungus, which infects the noses and wings of the animals, targets the bats while they're hibernating. It rouses them from their torpor more frequently than usual, causing the critters to use up too much of their stored fat, and then they waste away.
The fungus is present in both Europe and America, but old world bats seem to survive it much better, so it hasn't been clear if was a deadly American mutation, or if the fungus was only recently introduced to the population here.
To solve this riddle, scientists exposed a group of American bats to isolated versions of the fungus from each continent. Regardless of which type of isolate they were inoculated with, the results were the same: the bats woke up from their light-hibernation three to four times as often as the controls, and as a result were severely emaciated after a season.
While this still doesn't give us an effective way of treating the disease, what it does do is confirm the origins of the disease, and add more evidence to how we think it works. Now, we just need to fight it.